The present invention relates to a mechanism for rotatably fixing a disc to a rotary driving spindle for recording information on the disc or reproducing information previously recorded on the disc. Discs of this type include ordinary phonograph records, digitally recorded phonograph records, and video discs. The invention relates more particularly to a mechanism for fixing discs, especially discs which must be rotated at high speed, to a driving spindle, which mechanism is capable of accommodating discs having different center hole diameters.
It is possible to reproduce information from both a digitally recorded audio disc and a digitally recorded video disc using the same basic playback apparatus (of course using different demodulators). However, most conventional disc playing apparatuses are not capable of selectively mounting an audio disc or a video disc on the same spindle because the two different types of discs have significantly different center hole diameters.
In Japanese Utility Model Publication No. 180713/79 there is disclosed a disc clamping mechanism which can accommodate discs of different diameters. However, if the diameter of the disc being played deviates even slightly from one of the specified diameters, the disc will not being firmly attached to the spindle, making its reproduction flawed.
Another problem of the prior art disc clamping mechanisms arises in playing double-sided video discs. In manufacture, these discs are ordinarily formed by bonding together two prerecorded sides. It is difficult to perform this operation with such an accuracy that the tracks on both sides are precisely centered around the center hole. Thus, even if the disc is clamped so that information on one side can be reproduced correctly, the track on the other side may be so far off center that accurate reproduction of the other side is impossible.
Still another problem is involved in the fact that on a compact optically and digitally recorded disc the innermost diameter of the recorded track is not much greater that the diameter of the center hole, that is the innermost portion of the recorded track is closely adjacent the edge of the center hole in the disc. This effectively limits the dimensions of the clamping mechanism, making it difficult to obtain an adequate clamping force with the available space between the edge of the center hole and the innermost recorded track.
Prior art clamping mechanisms are shown in FIGS. 1 and 2 of the accompanying drawings. In these figures, A indicates a disc of a first thickness and center hole diameter (FIG. 1); A.sub.1, the center hole of the disc A; B, a disc of a second thickness less than the thickness of the disc A and center hole diameter less than the center hole diameter of the disc A; B.sub.1, the center hole of the disc B; and C, a spindle which is rotated by a motor (not shown). A turntable D, fixed to the spindle C, is provided at its center with bosses E and D, corresponding in outer diameters to the diameters of the center holes A.sub.1 and B.sub.1, respectively. The height of the boss E is equal to the difference in thickness between the discs A and B. An upward extension of the spindle C is threaded, and a clamper G is threadedly engaged with the threaded extension of the spindle C to clamp the disc between the clamper G and the turntable D. To mount a disc, the disc is placed on the turntable around the appropriate boss, and the clamper G is screwed onto the threaded extension of the spindle C.
If, as mentioned above, the center hole diameter of a disc being played does not precisely match the diameter of one of the bosses E and F, it is impossible always ensure that the disc is mounted on the turntable properly centered. Moreover, if it is desired to play a double-sided optical disc, frequently it is necessary to provide a separate centering member to center the tracks on the two sides of the disc. This makes the mounting of the disc complicated and time consuming. Hence, the mechanism is not suitable for use with video discs which require precise tracking.